1. Field of the Invention
The present invention relates to a liquid crystal display device, or more particularly, to an active matrix type liquid crystal display device.
2. Description of the Related Art
The active matrix liquid crystal display device has a plurality of gate signal lines, a plurality of drain signal lines, and pixel fields. The plurality of gate signal lines is extended in an x direction on a surface of one of two transparent substrates, which are opposed to each other with a liquid crystal layer between them, on the side of the liquid crystal layer. The plurality of gate signal lines are juxtaposed in a y direction orthogonal to the x direction. The plurality of drain signal lines is extended in the y direction and juxtaposed in the x direction while electrically isolated from the gate signal lines. The pixel fields are each defined by one pair of gate signal lines and one pair of drain signal lines. Arranged in each pixel field are a thin film transistor to be driven with application of a scanning signal over one of the pair of gate signal lines, and a pixel electrode to which a video signal is applied over a drain signal line via the thin film transistor.
In the foregoing structure, the pixel electrode is connected to one of the electrodes of the thin film transistor (source electrode) through a contact hole bored in an insulating film interposed between the electrode of the thin film transistor (source electrodes) and the pixel electrode. Part of the pixel electrode is extended to the other gate signal line. A capacitive element is formed using the insulating film interposed between part of the pixel electrode and the gate signal line as a dielectric. The capacitive element has the ability to hold (maintain) the video signal, which is applied to the pixel electrode, for a relatively long period of time even when the thin film transistor is turned off (becomes nonconducting).
However, the liquid crystal display device having the foregoing structure has a drawback that the gate-source capacitance of the thin film transistor in each pixel field cannot be set to a fixed value and the luminance of a display image therefore becomes inhomogeneous. Studies made in efforts to look into the causes of this drawback have demonstrated that: when the source electrode of the thin film transistor formed as part of the contact reaching the pixel electrode is formed through selective etching, the source electrode is unexpectedly hollowed and an area by which the source electrode is in contact with a semiconductor layer cannot be set to a fixed value.
Moreover, a variation in luminance appears at a point in each pixel which coincides with a capacitive element in each pixel field. Studies made in efforts to look into the causes of this drawback have demonstrated that an electric field is induced in parallel with a substrate between a gate signal line, which overlaps a pixel electrode, and the pixel electrode, and then invades into a substantial pixel field.
The present invention attempts to break through the aforesaid situation. An object of the present invention is to provide a liquid crystal display device capable of avoiding inhomogeneous luminance caused by a source electrode of a thin film transistor.
Another object of the present invention is to provide a liquid crystal display device capable of avoiding inhomogeneous luminance caused by a capacitive element.
A brief description will be made of representative aspects of the present invention below.
(1) A liquid crystal display device in accordance with the present invention has a semiconductor layer formed on a gate signal line with an insulating film between them, and a thin film transistor formed on the semiconductor layer, and having a first electrode connected on a drain signal line, and a second electrode connected to a pixel electrode. The second electrode has an extension that extends beyond the semiconductor layer and is connected to the pixel electrode through the extension. Moreover, the second electrode fans out on the semiconductor layer.
In the thus structured liquid crystal display device, the second electrode formed on the semiconductor layer have a sufficient unoccupied space preserved outside its contour. It therefore does not take place that an etching solution stagnates during formation of the second electrode through selective etching. Etching can therefore be performed according to a predetermined pattern. Consequently, the capacitance preserved between the gate electrode of the thin film transistor and the first electrode can be set to a fixed value. Eventually, the inhomogeneous luminance can be avoided.
(2) Moreover, a liquid crystal display device in accordance with the present invention has a pixel electrode formed in a pixel field defined on one of transparent substrates opposed to each other with a liquid crystal layer between them. Moreover, a black matrix for defining pixel fields is formed on the other transparent substrate. The pixel electrode is overlapped a signal line, which is formed in a portion of the one transparent substrate coincident with the black matrix, with a dielectric between them, whereby one of conducting surfaces of a capacitive element is formed. The width of the overlap is determined so that an electric field induced between the signal line and pixel electrode will not invade into the adjoining pixel field defined with the black matrix.
In the thus structured liquid crystal display device, the electric field induced between the signal line and pixel electrode will not serve as a noise to adversely affect the pixel field. Consequently, the variation in luminance will not take place.
Other objects and structures of the present invention, and operations and advantages to be exerted by the present invention will be apparent from the description to be made in conjunction with embodiments.